1. Technical Field
This invention relates to the techniques for eliminating aberration in optical systems and, more particularly to an optical element employing aspherical and binary grating optical surfaces.
2. Discussion
An image produced by an optical system has imperfections, called aberrations. Aberrations can take many forms. For example, chromatic or color aberration is a consequence of dispersion. Shorter wavelengths are bent the most, and focus nearest the lens, while the longer wavelengths focus farther from it. Spherical aberration results when spherical lens surfaces are used. Light striking nearer the periphery focuses closer to the lens while light striking near the center focuses farther away. Spherocromatism (or spherochromatic aberration) is a type of spherical aberration in which the focal points of light rays vary with the wavelength of the light. Finally, coma is an aberration that distorts images formed by off-access light rays that do not strike the lens and its center.
Conventional optical systems often require additional lenses to correct for aberrations, thereby adding cost, weight, size, and/or complexity. For example, most lenses designed for astronomical purposes consist of two elements of different indices of refraction The curvatures of the spherical surfaces of the elements are designed to reduce both the spherical and chromatic aberrations to a tolerable amount. Without corrective lenses, conventional optical systems would be limited by chromatic aberration to low speeds or focal ratios.
One alternative to adding lenses to correct for aberrations is the use of diffractive surfaces, such as one of many types of computer-generated Fresnel zone plates on at least one side of a lens. A high efficiency binary approximation of a Kinoform-type Fresnel zone plate, called a binary grating surface, has been disclosed for use in infrared systems by Swanson and Veldkamp in "Infrared Applications of Diffractive Optical Elements", SPIE Proceedings, Vol 885, paper #22, 1988. They further disclose the use of such a surface with a conventional or spherical lens. The spherical lens surface provides focusing, and the diffractive surface corrects for as much of the spherical aberration as is possible However, the use of a binary grating surface for correction of spherical aberration results in a significant amount of spherocromatism. Furthermore, since low speeds or focal ratios are required to limit this chromatic aberration, the usable optical speed is significantly limited.